Grease a 28 x 18 cm (11 x 7 inch) baking tin.
Mix together all the ingredients in a large bowl until well combined and spoon the mixture into the prepared tin.
Place in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes.
Remove from the oven and leave to cool in the tin. Mark into 15–20 squares and chill until firm. Store in an airtight container and eat wthin 3–5 days.